Modern commercial and military aviation applications often require communication systems to transmit high power RF signals in the presence of relatively small RF receive signals. In fact, there is a growing demand in the commercial aircraft industry to increase the number of radios present on a given platform. Similarly, the defense industry is constantly increasing the required number of signals to be simultaneously transmitted and received. Given the limited amount of space available on most platforms, it is therefore easy to understand that high power transmit antennas may interfere with nearby receive antennas. In fact, a typical transmit antenna will radiate hundreds or thousands of watts of power, whereas the power of the desired receive signal will be a fraction of that. If the receive antenna is located in relatively close proximity to the transmit antenna, residual transmitted power will be coupled to the nearby receive antenna. The result is saturation of the low noise amplifier (LNA) associated with the receive antenna. While the common sense approach to this problem is to physically separate the receive antenna from the transmit antenna, on platforms such as aircraft, helicopters, spacecraft, ships, and building tops, such a solution may not be possible due to limited space. Another solution is to use a cosite interference rejection system to cancel the coupled power from the interfering coupled signal obtained by the receive antenna.
A modern day interference rejection system is shown in FIG. 1 at 20. Generally, it can be seen that a transmit system 24 amplifies an input signal with a power amplifier 28 for transmission with a transmit antenna 21. The transmit signal is commonly sampled by a 10 dB coupler 23 for use by an interference subsystem 22. The interference subsystem 22 amplitude and phase weights the sampled transmit signal based on a feedback signal such that the weighted signal is effectively out of phase with the sampled transmit signal. A cancellation coupler 29 couples the weighted signal to an interfering coupled signal obtained from a nearby receive antenna 25. It is important to note that cancellation occurs in the electrical domain. Thus, the cancellation coupler 29 functions as an electrical cancellation subsystem. A feedback loop 26 provides the feedback signal to the interference subsystem 22 based on the desired receive signal produced by the cancellation coupler 29. The feedback loop 26 typically uses a feedback coupler 27 to effectively sample the desired receive signal. The desired receive signal is then passed on to an LNA 15 for amplification.
While the above described conventional interference rejection system 20 partially addresses the issue of cosite interference, there is still room for considerable improvement. For example, the conventional interference rejection system 20 is limited in the amount of coupled power that can be cancelled. In fact, when the coupled power exceeds the threshold of the rejection system 20, the system 20 can no longer transmit and receive simultaneously. The result can be a loss of information. This problem is generally due to the non-linearity of the electrical components used in the system 20. Specifically, the exact reduction in amplitude of the interfering signal depends on how accurately the phase and amplitude of the weighted signal matches the interfering signal. The combination of a high level interfering signal and loss in the couplers 23, 27, 29 makes it difficult for the interference subsystem 22 to maintain linearity. When the linearity degrades, the cancellation performance may be reduced. Eventually, as the interfering levels increase, large signals will reach the input to the LNA 15 causing saturation and additional non-linearities. Under these conditions, it is not possible to receive low-level signals near the system noise floor, and information will be lost. It is therefore desirable to provide a cosite interference rejection system that does not fall subject to the non-linearities associated with high level interfering signals.
Another concern relates to applications where weight distribution is important. For example, it is well known that conventional interference rejection systems can significantly effect the distribution of weight on modern day aircraft. In fact, it is quite difficult to arrange the components of the rejection system to redistribute weight towards the center of gravity in order to improve performance of the aircraft. This is largely due to the electrical nature of the components and connections associated with conventional interference rejection systems. It is therefore desirable to provide a cosite interference rejection system that allows for more efficient weight distribution.